1. Field of the Invention
The present invention relates to a piezoelectric film element, which is applied to a liquid discharging head of a liquid discharge recording apparatus such as an ink jet printer, and the like, a sound generating body such as a microphone, a speaker, and the like, various vibrators and oscillators, and further a drive unit, and the like for various sensors, to a method for manufacturing the piezoelectric film element as well as to a liquid discharge head.
2. Related Background Art
Heretofore, a piezoelectric film element using a dielectric thin film (piezoelectric film) having a piezoelectric property is widely used in sensors and actuators as an energy transforming element for transforming electric energy to mechanical energy. In particular, recently, there is a piezoelectric actuator of a liquid discharging head mounted on an ink jet printer, and the like as a distinguishing example to which the piezoelectric film element is applied.
The piezoelectric film element of the piezoelectric actuator of the liquid discharging head is generally composed of a piezoelectric film and an upper electrode and a lower electrode disposed across the piezoelectric film. As a component of the piezoelectric film, there are known a two-component composition mainly containing lead zirconate titanate (PZT) or a three-component composition prepared by adding a third component to the two components. Further, as a method for manufacturing the piezoelectric film, a sputtering method, a sol-gel method, a CVD method, and the like are used.
Japanese Patent Application Laid-Open No. H8-078748 discloses a piezoelectric film mainly containing PZT and an ink jet recording head.
FIG. 5 shows a film configuration of an ordinary piezoelectric film element disclosed in Japanese Patent Application Laid-Open No. H8-078748, in which the piezoelectric film element has a silicon substrate 101 as a substrate, a silicon thermal oxidized film 102, a lower electrode 103, a piezoelectric film 104, and an upper electrode 105, a close contact layer 105a is interposed between the upper electrode 105 and the piezoelectric film 104, and a close contact layer 103a is interposed between the lower electrode 103 and the piezoelectric film 104.
Although recent ink jet printers achieve image quality comparable with the quality of photographs, there are more increased needs of users for requiring a further improvement in image quality and an improvement in a print speed. To cope with these needs, a large amount of displacement as well as low power consumption and reduction in size are required for the piezoelectric actuator of the liquid discharge recording apparatus. Accordingly, it is desired to develop a piezoelectric film having higher piezoelectric characteristics. To obtain a good piezoelectric constant, a piezoelectric film must be ordinarily subjected to a heat treatment at a temperature from 500° C. to 700° C. or more. However, a conventional piezoelectric film element is made by laminating a silicon substrate, a piezoelectric film, a lower electrode, an upper electrode, and the like having a different thermal expansion coefficient laminated as shown in FIG. 5. Accordingly, stress is generated because they are thermally expanded differently in the heat treatment process described above due to a difference between their thermal expansion coefficients, thereby the piezoelectric property of the piezoelectric film is deteriorated, and cracks and the like occur. When the substrate is composed of a material having the same thermal expansion coefficient as that of the piezoelectric film, stress applied to the piezoelectric film can be reduced. However, a problem arises in that a material that can be used for the substrate and a manufacturing process are greatly limited. Further, stress can be also reduced by making the thickness of the substrate approximately equal to or less than that of the piezoelectric film. However, since the thickness of the piezoelectric film is made to several micrometers to several tens of micrometers, this is not practical although not impossible because handling is very difficult.
As an idea for reducing stress, Japanese Patent Application Laid-Open No. H10-095111 proposes a structure in which a lower electrode is composed of at least two metal film layers and the metal film particles of a first layer is made smaller than those of a second layer. With the above configuration, since the lower electrode curls in a direction opposite to that of a substrate, stress to a piezoelectric film can be reduced.
However, in the film configuration disclosed in Japanese Patent Application Laid-Open No. H10-095111, the thickness of the lower electrode must be made equal to or less than that of the piezoelectric film at least when the piezoelectric film is used as an actuator, and the thickness is ordinarily 0.5 μm or less. In contrast, since the thickness of the substrate is ordinarily 400 μm or more, it is contemplated that an effect for reducing the influence of thermal stress of the substrate by the lower electrode is small.